In a typical pneumatic transfer system, an article carrier is propelled between two spaced-apart terminals through an elongated transfer tube by air pressure. The carrier is propelled through the transfer tube by creating a pressure differential across the carrier. In other words, the carrier is propelled forward when a higher air pressure is created behind the carrier than exists in front of it. This may be accomplished by blower systems forcing air into the transfer tube behind the carrier thereby creating an increased pressure behind the carrier, or by reducing the pressure ahead of the carrier by creating a vacuum ahead of the carrier. In either situation, the efficiency of a pneumatic transfer system is directly related to the ability to minimize the air loss, i.e. air "bleed-by", around the carrier. To this end, "accelerator rings" are generally provided to surround and seal the periphery of a carrier to prevent air from "bleeding by" the body of the carrier.
An accelerator ring may be made from a wide variety of materials. The choice of materials used for accelerator rings are generally based upon the size of the carrier and the weight to be carried thereby. In this respect, soft, resilient materials, such as felt or belting, may be used for small diameter carriers transferring relatively small loads. Such materials adapt closely to the inner surface of the transfer tube and are pliable enough to conform to surface irregularities within the tube. Such materials, however, do not provide sufficient load carrying capability when used with large diameter carriers designed for transferring heavy loads, in that such materials wear too rapidly. Accordingly, a more rigid, wear resistent material is generally needed to support the weight of large weight-carrying carriers. For such applications, hard rubber or rubber reinforced leather belting is normally used. While such materials generally provide greater wear resistance, they are typically rigid and inflexible, and cannot readily conform to surface irregularities within the transfer tube. Therefore, when such a rigid material is used, the ring assembly must be dimensioned relative to the inner dimensions of the transfer tube so as not to bind the carrier therein.
In this respect, most transfer tubes are assembled from standard commercially available, metal or plastic tubes. Such tubes are available in various nominal sizes, but are subject to manufacturing tolerances which can vary the inner diameter of such tubes. Moreover, such tubes are generally not perfectly round. In this respect, most nominal sizes of tubing, especially metal tubing, have an "ovality factor", which is a dimension indicative of the degree or amount by which a cylindrical tube may be "out of round".
Accordingly, for large diameter heavy carriers having accelerator rings formed of rigid, generally non-flexible material, the outer diameter of such rings must be dimensioned to pass through a tube manufactured to the smallest tolerance and having the maximum ovality factor. In other words, the outer diameter of the accelerator ring must be dimensioned to pass through the smallest possible cylindrical opening the transfer tube may have as a result of manufacturing limitations.
As will be appreciated, when an accelerator ring is used within a transfer tube manufactured to the lower tolerance, a substantial gap will exist between the outer edge of the accelerator ring and the inner diameter of the transfer tube. Such a gap adversely affects the efficiency of the system by allowing air to "bleed-by" or flow past the carrier thereby reducing the pressure differential across the carrier and reducing the propelling force exerted on the carrier.
The present invention overcomes this and other problems by providing a seal arrangement for a carrier for use in pneumatic transfer system, which seal arrangement increases the efficiency of the transfer system by increasing the permissible size of the seal surrounding a carrier, and as a result reducing the annular gap created between the carrier and the inner surface of the transfer tube.